The present invention relates a semiconductor circuit, and more particularly to a semiconductor current switch circuit and a Digital-Analog Converter (hereinafter referred to as DAC) fabricated as an integrated circuit.
DAC's are used for converting a digitally coded signal to an analog signal and the theory of operation for DAC's is described in the book "Analog Integrated Circuit Design", by A. B. Grebene, Van Nostrand, 1972, p. 328-350.
Recently, a DAC employing a single resistor string provided with a plurality of intermediate taps and a plurality of MOS transistors arranged between the respective intermediate taps and an output terminal was proposed in "International Solid-State Circuit Conference" held in 1976 in Philadelphia, U.S.A. by Adib R. Hamade (Digest of Technical Papers pp. 154 to 155).
This DAC has a simplified circuit structure and is suitable for fabrication in the form of a monolithic integrated circuit. This DAC also provides an essentially monotonic input-output characteristic. However, since an output signal is derived from the resistor-division voltage at the intermediate tap through the MOS transistor, an output impedance is very large. Therefore, the output is difficult to use or utilize. Furthermore, a plurality of MOS transistors are connected between the intermediate tap and the output terminal and hence high operation speed cannot be obtained.
As another type of DAC, one employing an R-2R resistor ladder circuit has been utilized, which is disclosed, for example, in the U.S. Pat. No. 4,092,639. This type of DAC requires high accuracy in the resistor ladder circuit which is therefore usually fabricated with a trimmed thin resistive film. Therefore, it cannot be fabricated with ease. Furthermore, the stability in current output characteristic of current switch circuits employed in DAC's are not sufficient.